Modified pyrohydrolysis process for spent aluminum reduction cell linings

ABSTRACT

Fluoridic spent and waste materials, such as are generated in electrolytic aluminum reduction systems, are pyrohydrolyzed in a fluidized bed reactor. For fluidization of these materials, as well as for the combustion of carbon present in the materials, an O 2  -containing gas stream, containing at least about 90% by volume O 2 , is employed. This results in the generation of an HF-containing offgas of significantly increased HF level, which can be employed for the manufacture of an AlF 3  product of at least about 85% by weight AlF 3  content from hydrated alumina. The offgas having the increased HF content can also be employed for the production of highly concentrated HF acid with significantly lower energy requirement needed for concentration than in conventional systems.

BACKGROUND OF THE INVENTION

This invention relates to an improved pyrohydrolysis process for spentand waste materials generated in electrolytic aluminim reduction sytems.More particularly, it concerns a pyrohydrolysis process whereinfluidization of the fluoridic waste material charge is accomplished byuse of an O₂ -containing gas having an O₂ content of at least about 90%by volume.

Pyrohydrolysis of spent and waste materials generated in electrolyticaluminum reduction systems has been described in detail in copendingapplication Ser. No. 855,506 now U.S. Pat. No. 4,113,832 to Bell et al.The process described involves the pyrohydrolysis of spent aluminumreduction cell linings and other fluoridic waste materials, such asfloor sweeping, channel cleanings and spent alumina from reduction celloffgas scrubbing systems, in a fluidized bed reactor. Fluidization andpyrohydrolysis generates an NaF and HF-containing offgas from which NaFis recovered and the NaF-free gas, containing the Hf constituent, isused for AlF₃ manufacture and/or production of anhydrous HF. The clinkerproduct of the pyrohydrolysis is essentially Na₂ O.xAl₂ O₃, which isutilized as a source of Al₂ O₃ and Na₂ O, for example by the well-knownBayer process. In this process both NaF and HF are produced, however,the HF content of the offgas is relatively low due to the simultaneousgeneration of NaF and also since air is used as a fluidizing medium forthe spent material charge. Due to the diluteness of the HF in the gas,the AlF₃ product, which is made by contact of HF with Al₂ O₃ in aseparate fluidized bed reactor, has an AlF₃ content in the neighborhoodof about 13-20% by weight.

In copending application Ser. No. 910,416 to Andersen et al, filed May30, 1978, an improvement on the above discussed process is described. Inthe Andersen et al application, the pyrohydrolysis reactor, where thefluidization and pyrohydrolysis take place, is provided with an"extended" reaction zone in the freeboard area of the reactor. Theextended reaction zone, in which vapor-phase Na-containing compounds,such as NaF and Na₂ O, are contacted with a relatively finely dividedsource of Al₂ O₃ in the presence of steam, allows essentially completeconversion of the vaporized NaF to HF and also the formation of Na₂O.xAl₂ O₃ by the extended reaction of the vaporized Na₂ O with the Al₂O₃. This improved process provides an essentially NaF-free offgas inwhich the HF content is significantly increased due to the conversion ofthe NaF constituent to HF in the extended reaction zone. This higher HFcontent in the offgas renders the offgas more suitable for themanufacture of anhydrous HF and/or AlF₃ than previous processes.However, the HF is still considerably diluted due to the CO₂ -content ofthe offgas which has been generated by the combustion of the carboncontent of the waste materials, the excess water vapor required to drivethe pyrohydrolysis reaction to completion and the large volume ofnitrogen introduced with the air used for combustion.

It has now been found that the HF content of the pyrohydrolysis offgascan be further increased by a considerable degree using as fluidizingmedium and as a source for combustion of the carbon content of the wastematerial charge, a stream which contains at least about 90% by volumeO₂. Using the essentially nitrogen-free stream for these purposes, incombination with the expanded reaction zone concept where a relativelyfinely divided source of Al₂ O₃ is contacted with the vaporizedNa-components of the offgas, results in a more than 300% increase in HFcontent of the Na-free offgas. This offgas, as will be shownhereinafter, can be readily employed for the production of an AlF₃product containing at least about 85% by weight AlF₃ and/or a highlyconcentrated HF with a fraction of the energy required in comparison toprior art processes.

BRIEF SUMMARY OF THE INVENTION

An improved process for recovering fluoridic values fromcarbon-containing spent and waste materials, such as are generated inelectrolytic aluminum reduction system is provided by pyrohydrolysis ofthese materials in a fluidized bed reactor in the presence of steam. Inthe process, the fluidized bed reactor is provided with an expandedreaction zone above the fluidized bed in the reactor freeboard area bycontacting the vaporized Na-containing compounds, such as NaF and Na₂ Owith a relatively finely divided source of Al₂ O₃ in the presence ofsteam. This extended reaction provides an essentially NaF-free offgashaving an increased HF content. This HF content can be further increasedby the improved process of the invention wherein the medium introducedfor the fluidization of the waste material charge and for the combustionof its carbon content is an O₂ -containing gas of at least about 90% byvolume O₂ content. The reduction of the N₂ diluent from this fluidizingand combusting medium increases the HF content of the offgas by asignificant degree and permits the production of an AlF₃ product of atleast about 85% by weight AlF₃ content. Due to the significantlyincreased HF content of the offgas, manufacture of highly concentratedHF can proceed with considerably lower energy input than required forsystems employing air as fluidizing and carbon combusting medium.

BRIEF DESCRIPTION OF THE FIGURE

The FIGURE schematically shows the pyrohydrolysis of spent and wastematerials generated by electrolytic aluminum reduction systems. The HFgenerated by pyrohydrolysis in the fluidized bed reactor and theextended reaction zone is, after cooling and conventional dust removal,introduced in a tail gas scrubber for purging steam and other inertgases. From this scrubber high concentration HF is recovered which isused as a direct contact cooling medium, thus increasing the HF contentof the gas stream. The concentrated HF gas stream is then used for theproduction of an AlF₃ product of 85% AlF₃ content. It can also be usedfor the preparation of highly concentrated aqueous HF and ultimatelyanhydrous HF.

DETAILED DESCRIPTION OF THE INVENTION

An improved process is provided for the pyrohydrolysis of fluoridicspent and waste materials generated in electrolytic aluminum reductionsystem. More particularly, the process relates to the pyrohydrolysis ofcarbon-containing, fluoridic spent and waste materials in a fluidizedbed reactor, wherein an extended reaction zone is provided in thereactor freeboard area by introduction of a relatively finely dividedsource of Al₂ O₃ into this area, and by using a gas, containing at leastabout 90% by volume O₂, for the combustion of the carbon content and forthe fluidization of the spent and waste material charge.

For the purposes of this invention "pyrohydrolysis" of the fluoridicspent and waste materials of electrolytic aluminum reduction systemsrefers to the following reactions:

    2NaF + H.sub.2 O ⃡ 2HF + Na.sub.2 O            (1)

    2alF.sub.3 + 3H.sub.2 O ⃡ 6HF + Al.sub.2 O.sub.3 (2)

    na.sub.2 O + xAl.sub.2 O.sub.3 → Na.sub.2 O.xAl.sub.2 O.sub.3 (3)

these reactions take place at elevated temperatures, generally aboveabout 900° C. The water required for pyrohydrolysis reactions (1) and(2) is usually introduced as liquid water, combined water or as steam tothe fluidized bed or as a combination of these.

The expression "spent and waste materials of electrolytic aluminumreduction systems" encompass among others, carbonaceous cell or potlinings which are recovered after their useful lives have expired.Typical composition of such pot linings is shown in Table I.

                  TABLE I                                                         ______________________________________                                        TYPICAL COMPOSITION OF SPENT POT LININGS                                      Elements                                                                              % by Weight Elements     % by Weight                                  ______________________________________                                        Al      16.1        Ca           1.4                                          F       10.5        Fe           0.8                                          Na      11.8        Si           0.7                                          Li      0.3         CN           0.2                                          C       32.1        0.sub.2,CO.sub.3,Cl, etc.                                                                  26.1                                         ______________________________________                                    

In addition to the spent cell linings, the charge to the fluidized bedreactor can also contain floor sweepings, trench and channel cleanings,as well as the spent alumina scavenger of reduction offgas scrubbingsystems. Typical compositions of these spent and waste materials areshown in Tables II and III.

                  TABLE II                                                        ______________________________________                                        TYPICAL COMPOSITION OF COMBINATIONS                                           OF CHANNEL AND TRENCH                                                         CLEANINGS WITH FLOOR SWEEPINGS                                                Element          % by Weight                                                  ______________________________________                                        Al               32.0                                                         F                25.5                                                         Na               13.5                                                         Fe                1.0                                                         Si                0.4                                                         Others           Balance                                                      ______________________________________                                    

These spent and waste materials are generally admixed prior to chargingthe materials into the fluidized bed reactor.

In the instant invention an "extended" reaction zone, such as fullydescribed in copending application Ser. No. 910,416, filed May 30, 1978(Andersen at al), is employed. This "extended" zone is generated in thefreeboard area of the fluidized bed reactor by introducing a relativelyfinely divided source of Al₂ O₃ in the reactor. As shown in detail inthe above-referenced applicaton, the source of Al₂ O₃ can be introducedinto the reactor by adding it to the charge, or in the vicinity of thefluidized bed surface, or in a split stream or both. The size of therelatively finely divided source of Al₂ O₃ is kept in the range of about40-500 microns, this relatively small particle size allows elutriationor "lifting out" of the Al₂ O₃ from the fluidized bed when admixed withthe charge. It provides a reactive surface for reaction with thevaporized Na-containing constituents of the pyrohydrolysis offgas andalso due to its small size, will be heated rapidly in or in the vicinityof the fluidized bed thus avoiding cooling of the extended reactionzone.

The purpose of the introduction of the relatively finely divided Al₂ O₃source in the free board area of the fluidized bed reactor is to reactwith the Na₂ O formed in reaction (1) and thus shift the equilibrium infavor of the formation of HF, with simultaneous formation of Na₂ O.xAl₂O₃ according to reaction (3). This provides for the generation of anessentially NaF-free offgas and increased HF yield.

In order for the pyrohydrolysis reaction to proceed rapidly and to ahigh degree of completion, it is necessary to meet a series ofinterrelated criteria. Favoring the reaction rate and the degree ofcompletion of the pyrohydrolysis reaction are (1) elevated temperature(above about 900° C., generally in the range of about 900°-1300° C.),(2) efficient gas to solids contact, (3) the length of time thereactants are in the reaction zone, (4) the removal of intermediatereaction products (such as the combination of Na₂ O with Al₂ O₃ in theform of Na₂ O.xAl₂ O₃) and (5) the maintenance of a high partialpressure of water vapor in the reaction zone. Copending application Ser.No. 855,506 now U.S. Pat. No. 4,113,832 employs a fluid bed reactorusing air for combustion of the carbon in the criteria. Copendingapplication Ser. No. 910,416, provides an improved means of extendingthe reaction zone residence time, bettering the gas to solids contactefficiency and significant reduction of the volatile sodium compounds inthe offgases.

It has now been found that a further, significant improvement in thepyrohydrolysis process can be achieved by the use of a high oxygencontent stream in place of air for combustion. This improvement of theprocess, in conjunction with the use of an HF scrubber for the removalof excess steam and combustion products and the utilization of thegenerated aqueous HF for the cooling of the offgas by direct contact,can generate, as will be discussed in detail hereinafter, a gas streamwith an HF content sufficiently high to produce an AlF₃ product of atleast about 85% by weight AlF₃ content.

The use of a stream, containing at least about 90% by volume O₂, forcombustion of the carbon content of the charge eliminates the diluenteffect of the nitrogen associated with the common use of air for thispurpose. In addition, the heat load required to bring the nitrogencontent of the air to reaction temperature is eliminated, thus reducingthe heat input required for maintaining reactor temperature.Consequently, this use of a gas of high O₂ content also reduces thetotal combustion offgas volume and allows the partial pressure of watervapor be increased in the fluidized bed reactor. This enhances thepyrohydrolysis reaction, thus substantially increasing the HFconcentration in the offgas and also permitting the pyrohydrolysisreaction to proceed at lower operating temperatures. In turn, theincrease in offgas HF concentration results in increased fluidized bedreactor capacity which improves the operational efficiency of the entirepyrohydrolysis process.

The advantages obtained by the use of a fluidizing gas containing atleast about 90% by volume O₂ are manifold. Thus, for example, thehydrogen fluoride content of the offgas generated and removed from thepyrohydrolysis reactor, can be approximately 6-10% by volume. This highHF content offgas, particularly if it is directly cooled with a liquidof about 25% HF content such as is obtainable in the novel system of theinvention and as is shown hereafter, allows preparation of an AlF₃product of more than about 85% by weight AlF₃ content. This product canbe made by contacting the HF-containing gas with alumina trihydrate (Al₂O₃.3H₂ O) in a fluidized bed reactor. The utilization of aluminatrihydrate in the production of AlF₃ not only produces the desiredproduct, but it also generates steam which can be readily employed forthe pyrohydrolysis of a fresh charge of spent pot lining. Thisadditional feature of the invention results in significant energysavings since the steam requirement for pyrohydrolysis can be generatedin the system without the necessity of outside energy sources.

The operation of the novel pyrohydrolysis system, using oxygen as thefluidizing and combustion medium, will be further explained withreference to the FIGURE.

As shown in the FIGURE, the spent pot lining charge is introduced intothe pyrohydrolysis reactor where upon charging of oxygen (at least about90% by volume O₂ content) and steam, combustion of the carbonaceousconstituents, and generation of hydrogen fluoride containing offgas isachieved at temperatures in excess of about 900° C., usually within therange of about 900°-1300° C. In order to convert the volatilizedNa-containing components of the offgas, a relatively finely dividedsource of Al₂ O₃ is introduced either in the fluidized bed, or in the"extended reaction zone" or into both places. The offgases generated bythe combustion and pyrohydrolysis of the spent pot lining will containbesides HF, also steam and CO₂, as well as entrained solids. Due to theuse of O₂ for combustion, the diluting effect of nitrogen is minimizedand the HF-content of the offgas will be about 6-12% by volume. Afterremoval of the entrained solids the offgas is cooled. In an advantageousembodiment of the invention the hot gases exiting from the reactor canbe cooled directly with a cold stream of HF-containing gas, such asshown by the dotted lines in the FIGURE. Since this is a recycle streamand there is no heat removal from the system, the quantity of coolinggas used affects the intermediate concentration of HF, but has no effecton the HF concentration in the product gases. Subsequent cooling in adirect contact cooler, such as also shown in the FIGURE, with a highlyconcentrated HF stream, for example such as one recovered from the tailgas scrubber, can increase the concentration of the gas, for example upto about 13-16% by volume. The cooled HF-containing stream is thenintroduced into a scrubbing unit where water is employed as a scrubbingmedium. The scrubber offgas will be free of HF and will contain mainlywater vapor and CO₂. The aqueous HF stream recovered from the scrubberwill have an HF content of about 25% by weight. A portion of the steamdischarged from the scrubber unit may be recycled to the pyrohydrolysisunit as shown.

The aqueous HF recovered from the scrubber unit can be employed formaking anhydrous HF in addition to, as shown in the FIGURE, for thedirect cooling of the HF-containing offgas stream. The higher than usualHF-content of the aqueous stream allows its ready conversion toanhydrous HF by conventional means. When the aqueous stream is usedprimarily for direct cooling of hot offgas, a gaseous product which hasan increased HF content is obtained, for example about 13-16% by volume.

This stream, containing about 13-16% by volume of HF, is utilized ifdesired for the production of AlF₃. While the prior art generallyavoided the use of alumina trihydrate for the manufacture of AlF₃, theinstant process prefers the use of this starting material. In the priorart processes the water content of the hydrate would have posed problemssince the heat of reaction generated by the low HF content of the gasstream is insufficient to vaporize the water content of the trihydrateand to maintain the required temperature of the gaseous and solidreactants.

In the present process the by-product steam of the AlF₃ production isalso utilized. The steam discharged from the AlF₃ fluid bed reactor isused to provide a major portion of the steam requirement for thepyrohydrolysis reaction, thus providing a favorable heat and energybalance, surpassing the effectiveness of prior art systems. Use of theoffgas from the AlF₃ fluidized bed reactor as the source of steam forthe pyrohydrolysis reactor recycles trace quantities of unreacted HF.This increased the overall HF recovery and eliminates a possibleenvironmental problem. In addition, the aluminum hydrate startingmaterial is less costly than calcined alumina; also the AlF₃ product,due to its high, more than about 85% by weight, AlF₃ content, is aneconomically more valuable product than the prior art AlF₃ products ofabout 15% AlF₃ content.

Thus, it can be observed from the system shown that by using afluidizing and combustion promoting gas of at least about 90% O₂content, the entire pot lining recovery process can be made moreefficient both from a technical and an economical point of view.

The following example will provide further insight in the operation ofthe novel system.

EXAMPLE

Spent pot lining, having a composition shown in Table III was mixed withmiscellaneous spent and waste materials from the electrolytic aluminumreduction system. These materials included floor sweepings and channelcleanings and the composition of the combined charge is shown in TableIV.

                  TABLE III                                                       ______________________________________                                        COMPOSITION OF SPENT POT LINING                                               Elements                                                                              % by Weight Elements     % by Weight                                  ______________________________________                                        Al      14.8        Ca           1.3                                          F       13.8        Fe           0.7                                          Na      15.5        Si           0.7                                          Li      0.5         CN           0.3                                          C       29.6        O.sub.2, CO.sub.3, Cl, etc.                                                                22.8                                         ______________________________________                                    

                  TABLE IV                                                        ______________________________________                                        COMPOSITION OF FEED TO PYROHYDROLYSIS UNIT                                    Elements                                                                              % by Weight Elements     % by Weight                                  ______________________________________                                        Al      19.0        Ca           1.2                                          F       16.2        Fe           0.6                                          Na      14.8        Si           0.6                                          Li      0.3         CN           0.2                                          C       24.2        O.sub.2, CO.sub.3,Cl, etc.                                                                 22.9                                         ______________________________________                                    

The pyrohydrolysis reactor, operating at about 1200° C. was charged withthe feed at the rate of 4.61 t/h. In addition, a relatively finelydivided source of Al₂ O₃, containing 86.4% by weight Al₂ O₃ (asdetermined after heating to 1000° C. for about three hours) was alsoadded to the reactor at the rate of about 0.30 t/h. The particle size ofthe Al₂ O₃ source was in the range of about 200 to 400 microns.

A fluidizing and combustion supporting gas, containing about 95% byvolume O₂ was charged to the reactor at the rate of about 2440 m³ /htogether with steam which was introduced at the rate of about 14,400 m³/h. The offgas generated by the pyrohydrolysis was conducted throughconventional dust separators and the accumulated dust, consisting inessence of finely divided feed and Al₂ O₃ source, was recycled to feedpreparation. The offgas, which was freed of most of its entrained solidscontent, was then cooled to about 1000° C. by recycle of cooler gas andpassage through the superheater as shown in the FIGURE. The gas streamwas then conducted to a direct contact cooler where cooling of theoffgas to about 200° C. was accomplished by contact with an aqueousstream of about 25% by volume HF content. The cooled offgas, which nowhad an HF content of about 14% by volume, was then subjected to a finalsolids separation step, where essentially all of the dust was removed.The dust-free stream, containing HF 14 %, CO₂ 20% and steam 64% byvolume, was then separated into two streams. A stream was heated toabout 425° C. in the superheater and then employed for AlF₃ production;while the remaining portion was introduced in a scrubber unit in countercurrent flow to the aqueous scrubbing medium. The relative proportionsof these two gas streams were controlled in such a manner to utilize allof the HF content of the offgas stream to produce an AlF₃ product of85-90% by weight AlF₃ content at a production rate of 1.09 t/h. Thescrubbed offgas stream free of HF and consisting of CO₂ and water vaporwas released to the atmosphere, while the produced aqueous HF stream ofabout 25% HF content was used to directly cool HF-containing offgas fromthe reactor. The volume of cooling agent was controlled to obtain atemperature reduction of the offgas from about 1200° C. to about 200° C.

A portion of the steam effluent from the scrubber was introduced intothe pyrohydrolysis reactor.

The dust-free offgas stream of about 14% by volume HF content which wasdivided out of the main stream going into the scrubber was preheated toabout 425° C. in the superheater by indirect contact with incomingoffgas from the reactor, then it was used to make AlF₃ product in afluidized bed unit consisting of at least one and preferably of threeseries connected fluidized beds. The fluidized bed reactor was operatedat about 550° C. and Al₂ O₃.3H₂ O was charged to the reactor at the rateof about 1.20 t/h to provide for the above-mentioned production rate.The steam discharged from the reactor, after removal of entrainedsolids, was utilized for the pyrohydrolysis of the spent pot liningwhile the produced AlF₃ of about 85-90% by weight AlF₃ content wasemployed in the electrolytic aluminum reduction system as electrolyte.

The AlF₃ production unit does not have to operate continuously duringthe entire pyrohydrolysis process and production of AlF₃ can be madeoptional. If the AlF₃ production unit is operating at a reduced rate ordoes not operate, more, or all of the HF-containing offgas can beutilized for the generation of highly concentrated aqueous anhydrous HF.Thus, the instant system provides a highly desirable operatingflexibility.

While there have been shown and described hereinabove possibleembodiments of the invention, it is to be understood that the inventionis not limited thereto and that various changes, alterations andmodifications can be made thereto without departing from the spirit andscope thereof as defined in the appended claims.

What is claimed is:
 1. In the process of recovering valuable componentsfrom carbon and fluoride-containing aluminous spent and waste materialsgenerated in electrolytic aluminum reduction systems by subjecting thespent and waste materials to pyrohydrolysis in the presence of steam ina fluidized bed reactor having a fluidized bed and a freeboard areaabove the bed at a temperature within the range of about 900° C. toabout 1300° C., the improvement comprising the combination of stepsof(a) combusting the carbon content of the spent and waste materials inthe fluidized bed with an O₂ -containing stream having an O₂ content ofat least about 90% by volume thus generating a hot offgas substantiallyfree of nitrogen and containing HF and vaporized Na-containingcompounds; (b) establishing an extended reaction zone in the freeboardarea of the reactor by contacting the hot offgas with a relativelyfinely divided source of Al₂ O₃ having a particle size within the rangeof about 40 to about 500 microns whereby the Na-containing vapors areconverted to HF and Na₂ O.xAl₂ O₃ ; (c) cooling the hot, HF-containingoffgas by direct contact with an aqueous HF-containing stream andconducting the cooled offgas to a scrubber for the removal of its HFcontent by scrubbing with an aqueous medium, while generating a steamand inert gas containing scrubbed offgas; (d) recovering the aqueous HFfrom the scrubber and using at least a portion thereof for cooling hotHF-containing offgas generated in the fluidized bed reactor; (e)recovering an HF-containing side stream from the cooled HF-containingoffgas prior to scrubbing of the offgas with the aqueous medium andemploying the HF-containing side stream for the production of an AlF₃product of at least about 85% by weight AlF₃ content in a fluidized bedreactor using as alumina source alumina trihydrate; and (f) recoveringthe AlF₃ product and steam from the AlF₃ reactor and recycling the steamto the pyrohydrolysis reactor.
 2. Process according to claim 1, whereinthe temperature in the fluidized bed reactor is kept within the limitsof about 1000° C. and about 1200° C.
 3. Process of claim 1, wherein atleast a portion of the aqueous HF-containing stream is used for makinganhydrous HF.
 4. Process according to claim 1, wherein all of theaqueous HF stream recovered from the scrubber is employed for coolinghot HF-containing offgas by direct contact.
 5. Process according toclaim 1, wherein the HF content of the hot offgas generated in thepyrohydrolysis reactor is about 6-10% by volume and after direct coolingwith the aqueous HF-containing stream from the scrubber the HF contentof the offgas increases up to about 13-16% by volume and this gas issubjected to scrubbing with an aqueous medium.
 6. Process according toclaim 5, wherein scrubbing of the offgas with an aqueous mediumgenerates an aqueous HF containing up to about 25% by weight HF. 7.Process according to claim 1, wherein the HF-containing side stream,used for the production of an AlF₃ product of at least 85% by weightAlF₃ content in a fluidized bed reactor, is preheated to the temperaturerequired for production of AlF₃ by heat exchange through indirectcontact with the hot offgas generated in the fluidized bed reactor.